To be more precise, the UUID for Xcode was already there in info.plist.

The workaround did not work for me

Workaround for whenever in the future this might happen again:

- Open Applications in Finder
- right-click XCode app > Show package contents
- open Contents
- open Info.plist
- copy the DVTPluginCompatibilityUUID string

- In the Finder window, press ⇧⌘G (Shift+cmd+G)
- Enter /Applications/Unity/PlaybackEngines/iOSSupport/Tools/OSX/Unity4XC.xcplugin/Contents
- open Info.plist in your favorite text editor
- find the part with:
<key\>DVTPlugInCompatibilityUUIDs</key>
<array>
<string>63FC1C47-140D-42B0-BB4D-A10B2D225574</string>
<string>37B30044-3B14-46BA-ABAA-F01000C27B63</string>
<string> etc...

- add a line with:
<string>[Paste the copied UUID here]</string>
before the line with </array>

You'll be able to have Unity run the build process in XCode again (after restarting XCode)

Whenever I try I get this:

Unity xcode plugin has not current Xcode in its compatibility list.
Please launch the project manually
UnityEditor.BuildPlayerWindow:BuildPlayerAndRun()

Could not create a new Xcode project with CocoaPods: pod executable not found
UnityEngine.Debug:Log(Object)
GoogleMobileAds.Postprocessor:runPodUpdate(String) (at Assets/GoogleMobileAds/Editor/PostProcessor.cs:48)
GoogleMobileAds.Postprocessor:OnPostprocessBuild(BuildTarget, String) (at Assets/GoogleMobileAds/Editor/PostProcessor.cs:27)
UnityEditor.HostView:OnGUI()